test/docs(kit-webhook): integration tests, operator setup docs, PR-preview smoke test

[mei-artificerinnovations]

Background

Flagged as engineering follow-ups during Phase 4 and Brad's Phase 5a re-review. The kit-webhook Edge Function shipped in PR #340 (now closed) but the following were deferred.

Outstanding items

Integration tests

• End-to-end test covering the kit-webhook → enqueueMarketingEmail → queue table flow
• Non-admin rejection path coverage (pattern exists in waitlist.test.sql lines 114–122)

Operator setup documentation

• Docs for configuring the kit-webhook integration (Kit form ID, namespace, tier tag names)
• Runbook for verifying the webhook is receiving events correctly

PR-preview deploy smoke test

• PR-preview deploys do not currently exercise the kit-webhook Edge Function
• Add a lightweight smoke check to the deploy-preview pipeline

Context

Part of the Phase 5 (#286) marketing-email integration. kit-webhook implementation is in PR #340. Non-blocking fast-follows deferred from Phase 4/5a.